Re: [Xen-devel] Woes of NMIs and MCEs, and possibly how to fix

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.12.12 at 12:24, George Dunlap <dunlapg@xxxxxxxxx> wrote:
> On Fri, Nov 30, 2012 at 5:34 PM, Andrew Cooper 
>> As for 1 possible solution which we cant use:
>>
>> If it were not for the sysret stupidness[1] of requiring the hypervisor
>> to move to the guest stack before executing the `sysret` instruction, we
>> could do away with the stack tables for NMIs and MCEs alltogether, and
>> the above crazyness would be easy to fix.  However, the overhead of
>> always using iret to return to ring3 is not likely to be acceptable,
>> meaning that we cannot "fix" the problem by discarding interrupt stacks
>> and doing everything properly on the main hypervisor stack.
>>
> 
> 64-bit Intel processors have SYSEXIT, right?  It's worth pointing out the
> following alternatives, even if we never actually use them:
> 
> 1. Use SYSEXIT on Intel processors and let the bugs (or some subset of
> them) remain on AMD
> 2. Use SYSEXIT on Intel processors and IRET on AMD

SYSEXIT isn't very suitable because you'd have to corrupt %edx,
i.e. it couldn't be used for hypercalls with just 1 or 2 arguments.

Plus our GDT layout doesn't match that needed by SYSEXIT, yet
some of the selector values are part of the ABI.

Jan


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